This invention relates to apparatus for measuring the alcohol concentration in the breath exhaled by a subject.
It is known that the alcohol concentration in the exhaled air is uniquely related to the alcohol concentration in the blood if the exhaled breath comes from the lungs. Therefore, the subject must exhale a volume of air at least equal to the volume of the mouth cavity and the windpipe before a valid measurement can be obtained. In known equipment, a rate sensor is positioned in the input channel receiving the exhaled breath and the rate sensed must exceed a certain minimum rate for a predetermined period before the value of alcohol concentration as furnished by a gas detector is considered a valid measurement. This arrangement has the disadvantage that the minimum rate must always be exceeded, even so for persons whose normal rate of exhalation is very low and from whose lungs a sample of air could be obtained at a slower rate.